It is practice to convert printed media into images in these years. Such information converted into images can be electronically handled using terminal devices or the like.
PTL1 describes a mixed media document system which generates a mixed media document including at least two media (for example a printed sheet of paper as a first medium and a digital content as a second medium). The system described in PTL1 includes a content-based search database constructed with an index table and searches for a content using a text-based index. In the system described in PTL1, two-dimensional geometric positional relations between objects extracted from a printed document are stored in the index table and candidate documents are calculated from the index table on the basis of given data.
PTL2 describes a method for extracting feature points from an image. PTL3 describes calculation of the ratio between the areas of two connected regions as an invariant to affine transformation.
PTL4 describes an image search device that searches for an image including a region having a feature similar to that of a search key image. The image search device described in PTL4 extracts feature quantities from multiple regions in a registered image and compares the extracted feature quantities with a feature quantity extracted from a search key image to find a similar image that includes a region similar to the search key image.